Aiming at improvement on productivity in a die-bonding process, in the production of semiconductors, the in-line curing method in which a die bonder, a wire bonder and the like are arranged on the same line is employed and it is tending to be more widely used from now on. Hence, in comparison with a curing condition of a die attach paste in the conventionally employed batch method, time required for curing is significantly limited. For example, with respect to the conventional oven curing method wherein curing is performed at 150 to 200° C. for 60 to 90 minutes, curing is required in a short time such as at 150 to 200° C. for 15 to 90 seconds in the case of the in-line curing method (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2000-63452).
With the enhancement of the processing speed of semiconductor devices, a chip surface layer which is weak in mechanical strength is growing popular. In order to decrease stress against the chip surface, it is required to minimize warpage of a semiconductor chip due to the difference in thermal expansion coefficient between the semiconductor chip and a copper frame. Also, it is required to prevent oxidation of the copper frame. Thus, for complying with these requirements, the curing process in low temperature is desired.
In relation with the above requirements of shortening of curing time and lowering of curing temperature, there is another problem, which is a requirement of thermal management. With significant increase in capacity and processing speed of semiconductor products and moving towards finer design rule, a problem of heat which generates during operation of semiconductor products is becoming prominent so that releasing the heat from semiconductor products, i.e. thermal management, is becoming an important issue. Hence, measures to mount heat dissipating members such as heat spreaders, heat sinks or the like on semiconductor products are generally employed. However, it is desired for material itself which bonds heat dissipating members to have higher thermal conductivity.
On the other hand, some forms of semiconductor products may allow a semiconductor chip itself to bond to a heat spreader made of metal, a heat spreader to bond to a die pad portion of a lead frame to which a semiconductor chip is also bonded, or a die pad portion to expose at the package surface so as to serve as a heat sink as well. Further, a semiconductor chip may be bonded to an organic substrate having heat dissipating mechanism or the like such as a thermal via or the like. High thermal conductivity is also required to the material which bonds a semiconductor chip in such a case. In this manner, high thermal conductivity is required to die attach materials or materials for bonding heat dissipating members. At the same time, the die attach materials and the materials for bonding heat dissipating members are required to endure a reflow process performed when a semiconductor product is mounted on a substrate, often required to have bonding in large area, and also required to have low stress property in order to suppress warpage or the like due to different thermal expansion coefficients among component members.
However, a highly thermal conductive adhesive has some problems as follows (For example, see JP-A No. Hei. 11-43587). Generally, thermally conductive particles such as metal fillers such as silver powder, copper powder or the like, or ceramic fillers such as aluminum nitride, boron nitride or the like are added to an organic binder of the highly thermal conductive adhesive at high containing rate. However, since there is a limit in containing amount, there are cases that high thermal conductivity cannot be obtained. Also, though the highly thermal conductive adhesive containing a large amount of solvent has a good thermal conductivity as a sole cured product, the state when it is applied to a semiconductor product may not have a stable thermal conductivity since the solvent remains in the cured product or the solvent volatilizes to leave voids. Even if the adhesive could contain a large amount of thermally conductive particles, there are cases that low stress property is insufficient due to high containing rate of the thermally conductive particles.
On the other hand, as a part of considerations for the environment, since it has become to use a lead-free solder as a solder for mounting on a substrate, reflow temperature needs to be raised compared to the reflow temperature when Sn—Pb solder has been used. Because the use of such a lead-free solder causes increase of stress due to raised reflow temperature, semiconductor products easily generate delamination and thus cracking during the reflow process. Therefore, even higher reflow resistance (high reflow reliability) is demanded to constituent materials of semiconductor products than ever.
Further, at present, abolition of lead from semiconductor products is in progress as a part of considerations for the environment. The case is increasing to change plating of lead frames to Ni—Pd plating in order to exclude lead from outer lead plating of semiconductor products. In case of Ni—Pd plating lead frame, there is usually very thin gold plating (gold flash) on the surface to be stable. However, adhesion strength of a surface decreases in comparison with a normal copper frame with silver plating or the like because of smoothness of Ni—Pd plating and presence of gold on the surface. Thus, there are cases that a surface of Ni—Pd plated frame is chemically or physically roughened to improve the adhesion strength. However, such a roughened surface often causes resin bleeding of die attach paste which leads to serious problems such as decline in package reliability.
In relation to the above-mentioned problems, in case of epoxy resin type die attach paste, which is a major die attach paste at present, curing can be performed in about 60 seconds by using amine type curing agents or the like, however, curing in a very short time such as 15 to 30 seconds is not handled yet.
On the other hand, it is known, for example, in the following arts that adhesion particularly with metal improves by using compounds having an imide group such as maleimide or the like. That is, Japanese translation of PCT international application (JP-T) No. Hei. 10-505599, JP-T No. 2000-514496, JP-T No. 2001-501230, JP-A. No. 11-106455, JP-A No. 2001-261939 and JP-A No. 2002-20721. However, there are disadvantages that water absorption property of a cured product becomes high when an imide compound is used solely so as to deteriorate properties of a cured product after moisture absorption since imide compounds have high polarity. Also, when it is used together with other components, it is necessary to add components having high polarity in order to mix uniformly. As for these added components, similarly as above, properties of a cured product after moisture absorption deteriorate. There has not been any satisfactory compound particularly in view of adhesion to hardly adhesive surfaces such as Ni—Pd plated frames or the like, low stress property against increase of stress caused by raised reflow temperature due to the change to lead-free solder, and moisture resistance.
Also, a material which has better adhesion to Ni—Pd plated frames than conventionally used die attach pastes (for example, see JP-A No. 2000-273326), has excellent low stress property in terms of low elastic modulus, and does not generates resin bleeding is desired, however, there has not been satisfactory material.